Why were the continents once all piled in a big clump?

July 31, 2009

Dear Cecil:

I've seen pictures of Pangaea, the giant land mass that eventually separated into the continents we know today. But why were the continents smushed together like that in the first place? What made the land higher on that one side of the earth? Were there other continents we can no longer account for? Is it related to the asteroid that may or may not have smashed into the earth and helped form the moon?

— Chris D., Cranston, Rhode Island

Cecil replies:

Careful, bud. Thinking outside the box is great, but we don't want to cross the border into the completely insane. That's a chronic risk with continental drift, talk of which was a sure way to clear out your end of the bar at scientific conferences until the 1950s and which still inspires wacky theories. Asteroids don't figure in any of those I've heard about — but wait till you get a load of the expanding earth.

The most famous early proponent of continental drift, German geophysicist Alfred Wegener, was received skeptically when he proposed his theory in 1912, partly because he couldn't explain what might cause giant landmasses to move around. Expanding-earth advocates thought they could. They posited that once upon a time the earth had been much smaller and was completely encased in the supercontinent we now call Pangaea. Volcanic activity caused the planet to expand, cracking Pangaea apart like the shell of a boiled egg and leading to the eventual scattering of the continents.

Obvious objection: Where was all the extra volume that went into the expanding earth supposed to be coming from? Was the earth rising like a cake in the oven? Some proponents claimed the expansion was a result of a reduction in the universal gravitational constant or of the creation of new matter in the planet's core by some strange subatomic process; others just insisted by various proofs that the earth was expanding for reasons unknown. But it wasn’t, and isn’t. Precise measurements have now established that the earth hasn't enlarged appreciably since the era of the dinosaurs. Claims to the contrary aren't taken seriously by scientists.

Or most scientists, anyway. I notice that one James Maxlow was awarded a PhD by a seemingly respectable institution in Australia on the strength of a 2001 thesis in which he claims, among other things, that the present-day continents fit together with 99 percent accuracy if projected onto a smaller sphere. One admires Maxlow for his persistence in pursuing this notion. (He's got a book and a Web site, attends conferences, etc.) But one also remains pretty confident it's nuts.

If a once-smaller earth doesn't explain why the continents were all smushed together at one point, what does? We'll get to that. The main thing to understand is that the earth has been in a constant if extremely slow froth for much of its 4.6 billion-year existence — Pangaea, thought to have existed 250 million years ago, wasn't the first supercontinent and won't be the last. Conjectured predecessors include Ur (3 billion years ago), Kenorland (2.7 to 2.5 billion), Columbia (1.9 to 1.8), Rodinia (1.1), and Gondwana (540 million years ago). The constant shuffling arises from the fact that the hard outer shell of our planet floats atop a region of flowing molten rock, allowing the continents to skate along at the rate of 1 to 2 inches per year. The chief engine of plate tectonics, as this process is called, is the seafloor. At the midocean ridges, molten rock pushes up from below, causing the floor to expand laterally. Meanwhile, closer to the coasts, the edges of the floor get shoved below the continental plates in a process called subduction. Because of this, very little of the seafloor is more than 200 million years old, while parts of the continents are older than 4 billion years.

Why do we get supercontinents periodically? Some suggest that the continents are drawn together by zones in the earth where the seafloor is pulled down into the lower mantle in a process called superdownwelling, drifting toward the suction like rubber ducks in a draining bathtub till they collide. Why do supercontinents later break apart? One theory is that the oversize landmass traps so much heat beneath it that the crust ultimately cracks open. Another idea is that crust-rending "superplumes" of hot magma roil up from the spots where the superdownwelling occurred. Same result either way: the big continent splits back into smaller ones.

What next? I found maps offering one vision of the future on the Web site of Christopher Scotese, a geologist at the University of Texas at Arlington. The highlights: about 50 million years from now Africa plows into Europe, about 150 million years from now Australia becomes one with Antarctica, and by about 250 million years from now another supercontinent has formed, with North and South America, Eurasia, and Africa in one giant clump. In short, the earth will stay lively, not that it'll matter to us.